Improved Logical and Math support

Support logical functions;

• FALSE
• TRUE

Support math functions;

• ATAN2
• COSH
• DEGREES
• ACOS
• ACOSH
• ASIN
• ASINH
• ATAN
• ATAN2
• CEILING
• RADIANS
• COS
• COSH
• EXP
• EVEN
• FACT
• FACTDOUBLE
• INT
• LOG
• LOG10
• RAND
• RANDBETWEEN
• SIGN
• SIN
• SQRTPI
• TAN

0.2.12: Update changes.rst to include unary operator bugfix

Bugfix unary operator. It needed to be right associated to handle cases of double use eg; double-negative.. --4 == 4

Added support for functions;

• PV
• XIRR
• ISEVEN
• ISODD
• ISNUMBER
• ISERROR
• FLOOR
• ISERR

Roll version 0.2.11

Add functions:

• DAY
• YEAR
• MONTH
• NOW
• WEEKDAY
• EDATE
• EOMONTH
• DAYS
• ISOWEEKNUM
• DATEDIF
• FIND
• LEFT
• LEN
• LOWER
• REPLACE
• TRIM
• UPPER
• EXACT

0.2.10: Update setup.py and README.rst to comply with long_description_conten…

…t_type

0.2.4: Make a release

• Updated README with supported functions.

• Fix bug in ModelCompiler extract method where a defined name cell was being
  overwritten with the cell from one of the terms contained within the formula.
  Added a test for this.

• Move version of yearfrac to 0.4.4. That project has removed a dependency
  on the package six.

0.2.3

Merge branch 'in_board_xlfunctions'