summing together FmtStrs is slow - test says so
added tests for immutability we want to implement
Merge branch 'master' into immutable-FmtStrs

Conflicts:
	test.py
Made BaseFmtStr immutable.
Merge pull request #5 from oufeidong/immutable-FmtStrs

Immutable fmt strs
nicer gameexample
Merge branch 'master' of github.com:thomasballinger/fmtstr
Added test for copying FmtStr with new attributes.
Replaced FmtStr's set_attributes() method with copy_with_new_atts().