proper array index validation (dash, unsigned int)

According to the spec
http://tools.ietf.org/html/draft-ietf-appsawg-json-pointer-06

   o  If the currently referenced value is a JSON array, the reference
      token MUST contain either:

      *  characters that represent an unsigned base-10 integer value
         (possibly with leading zeros), making the new referenced value
         the array element with the zero-based index identified by the
         token, or

      *  exactly the single character "-", making the new referenced
         value the (non-existant) member after the last array element.
remove JsonPointer.set() to avoid duplicate code w/ python-json-patch

json-patch (https://github.com/stefankoegl/python-json-patch/) should be used
for modifying JSON documents
fix running tests during travis build
fix validation of array indices
fix installing when not in source directory, fixes #3
bump version to 0.6
no leading zeros for array indices, as of draft 08
unambiguous array indices make pointers comparable
bump version to 0.7
Extend test case to allow peeking an element from a list based on its content
Escape '[' and ']' special chars
implement access by filter to JSON Lists

Conflicts:
	jsonpointer.py
	tests.py
Handle not str cases