Change checkbox to create legends

The option to print the legends for the plots used to read "Create
Labels?". Now reads "Create legend" and has the checkbox to the left
like the other checkboxes in the GUI.

Fixes #199
Add new class to reflect unsupported operations

EmperorUnsupportedComputation is a sublclass of ValueError used to
comunicate the fact that a calculation cannot be performed with the
given inputs. Basically it prevents a less meaningful error from
happening.
Add usage of EmperorUnsupportedComputation

emperor.biplots now makes use of EmperorUnsupportedComputation to error
out in the case where a contingency table with one or fewer rows is used
to compute a biplots and their scores.
Add check for tables with one row

The script interface now verifies that the contingency table being
passed has more than one row in it, if this is not true then an error
message will be presented.

Fixes #184
Add note Fixing #184 in ChangeLog.md
Merge pull request #208 from ElDeveloper/issue_184

Fixes issue 184
Merge pull request #207 from ElDeveloper/issue_199

Change checkbox to create legends
Move use gradient colors to Colors tab

The checkbox to switch between coloring schemes (continuous or
discrete), used to be in the 'View' tab but is now found under the
'Colors' tab. The test module code was updated to reflect this changes.

Add a note to the ChangeLog.md file about this change.

Fixes #204
Merge pull request #209 from ElDeveloper/issue_204

Move "Use gradient colors" to Colors tab