Change comparable()'s arguments from number_1, number_2 to number1, number2

    * Keep argument naming in line with other functions
    * Tests updated to reflect changes
Change documentation to reflect new names of comparable()'s arguments
percentage arguments changed from value to number

* tests updated to reflect change
Documentation uses 'number' instead of 'value' for percentage()
Change round()'s argument from 'value' to 'number'

* Updated test_round to reflect change in argument name
* Exception in assert_error_message - I did not change "$value: #cccccc is not a number for `round'" because the name of the class is used to create the error message.
Change from 'value' to 'number' in documentation for round()
Change ceil()'s argument from 'value' to 'number'

* Update tests to reflect changes
Change documentation for ceil() - 'number' instead of 'value'
Change floor()'s argument from 'value' to 'number'

    * Update tests to reflect changes
Change documentation for floor() - 'number' instead of 'value'
Change abs()'s argument from 'value' to 'number'

    * Update tests to reflect changes
Change documentation for abs() - 'number' instead of 'value'
Change mix()'s arguments from 'color_1','color_2' to 'color1','color2'

  * Update tests to reflect changes
Change documentation for mix()'s arguments - 'color1','color2' instead of 'color-1','color-2'
Update changelog for comparable(), percentage(), round(), floor(), ceil(), abs(), mix()
Merge remote-tracking branch 'JonathanTR/issue-833'

Conflicts:
	doc-src/SASS_CHANGELOG.md
Add support for using the deprecated names for arguments.
Merge branch 'issue-833'

Closes #833