Merge pull request #19 from ebranca/master

merge dev and master
add math module
add string module
add seq module
add function clamp
new dcheck module
add WRONG_ONE_CHAR_STRING
add hard module
fix fpath variable and missed imports
add wrong byte value message
add rule for backup files '*~'
new intern module
add filter in Directory.ls() and add original path
fix ZeroDivisionError
add centroid, next_multiple, div_ceil
add module with utilities for pysec.io
add Rabin-Karp algorithm
add eq() and fix negative step in xrange()
fix doc
add functions to find common prefix and suffix, and newlines splitter