implemented a string formatter

Ex:
```
import fileseq

seqs = fileseq.findSequencesOnDisk("/Users/gregcotten/Temp/filesequence")
for seq in seqs:
	print seq.format("{dirname}{basename}[{range}]{extension} | Missing: {missing}", uniquePerFrameRange = True)
```

results in:
```
/Users/gregcotten/Temp/filesequence/test.[000-005].dpx | Missing: 006-010
/Users/gregcotten/Temp/filesequence/test.[011-099].dpx | Missing: 006-010
```