[MRG + 1] Create a new figure and test each plot type #127 (#179)

* [MRG] Create a new figure and test each plot type #127

 - move `plot()` to `plotting.py` as `plot_pdf()`
 - modify plotting functions to return matplotlib figures
 - add `test_plotting.py` and baseline images
 - import `plot_pdf()` in `__init__`
 - update `cli.py` to use `plot_pdf()`
 - update advanced usage docs to reflect changes

* Change matplotlib backend for image comparison tests

* Update plotting and tests
 - use matplotlib rectangle instead of `cv2.rectangle` in
`plot_contour()`
 - set matplotlib backend in `tests/__init__`
 - update contour plot baseline image
 - update `test_plotting` with more checks

* Update plot tests and config
 - remove unnecessary asserts
 - update setup.cfg and makefile with `--mpl`

* Add  to

* Add tolerance

* remove text from baseline plots
update plot tests with `remove_text`

* Change method name, update docs and add pep8

* Update docs

tests/__init__.py

@@ -0,0 +1,2 @@
+import matplotlib
+matplotlib.use('agg')

tests/test_plotting.py

@@ -0,0 +1,51 @@
+# -*- coding: utf-8 -*-
+
+import os
+
+import pytest
+
+import camelot
+
+
+testdir = os.path.dirname(os.path.abspath(__file__))
+testdir = os.path.join(testdir, "files")
+
+
+@pytest.mark.mpl_image_compare(
+    baseline_dir="files/baseline_plots", remove_text=True)
+def test_text_plot():
+    filename = os.path.join(testdir, "foo.pdf")
+    tables = camelot.read_pdf(filename)
+    return camelot.plot(tables[0], plot_type='text')
+
+
+@pytest.mark.mpl_image_compare(
+    baseline_dir="files/baseline_plots", remove_text=True)
+def test_table_plot():
+    filename = os.path.join(testdir, "foo.pdf")
+    tables = camelot.read_pdf(filename)
+    return camelot.plot(tables[0], plot_type='table')
+
+
+@pytest.mark.mpl_image_compare(
+    baseline_dir="files/baseline_plots", remove_text=True)
+def test_contour_plot():
+    filename = os.path.join(testdir, "foo.pdf")
+    tables = camelot.read_pdf(filename)
+    return camelot.plot(tables[0], plot_type='contour')
+
+
+@pytest.mark.mpl_image_compare(
+    baseline_dir="files/baseline_plots", remove_text=True)
+def test_line_plot():
+    filename = os.path.join(testdir, "foo.pdf")
+    tables = camelot.read_pdf(filename)
+    return camelot.plot(tables[0], plot_type='line')
+
+
+@pytest.mark.mpl_image_compare(
+    baseline_dir="files/baseline_plots", remove_text=True)
+def test_joint_plot():
+    filename = os.path.join(testdir, "foo.pdf")
+    tables = camelot.read_pdf(filename)
+    return camelot.plot(tables[0], plot_type='joint')