Addressed Tavis's comments in http://codereview.chromium.org/3023041/.

- Modified 4-bytes 16.16 Fixed-point number decoder in ReadNextNumberFromType2CharString(). 
- Fixed off-by-one error in ExecuteType2CharStringOperator(). 
- Fixed checks of 'div' and 'endchar' operators. They were not strict enough. 

- Wrote ~1600 lines of test to verify if the modification above is valid. 
- Moved Type2CharStringOperator enum definition from cff_type2_charstriung.cc tocff_type2_charstriung.h to write the unit tests. 

Review: http://codereview.chromium.org/3027049

BUG=51070
TEST=http://code.google.com/p/ots/wiki/HowToTestOts (Verified that the modified OTS rejects all malformed fonts, and does not reject all non-malformed fonts. I tested ~3400 TrueType and ~450 OpenType fonts.) 
TEST=ran cff_type2_charstring_test.