Added unit test for X-TAGs
Empty values are ignored now.
Validation changes:
- Attributes with less important values are cropped to maximum length (according to SPD specification)
- Attributes with important values are not cropped. For example: RF, X-VS, X-KS, X-SS are designed for various identifiers which are important for correct realization of payment. The crop is not allowed in this case.

Improved coverage in unit tests.
Added 'Library' folder to gitignore.
Now it's possible to create SmartPayment from NSData. The input data must be Latin-1 encoded.
X-PER value may be in interval <0,31>
Merge remote-tracking branch 'upstream/master'