Added descending branchlength method

Method allows calculation of descending branchlength in trees and replaces the cogent functionality totalDescendingBranchlength().
Added tests and resolved some pep8 problems
Updated tests and pep8 compliance
Final pep8 checks out
Pep8...
Resolve documentation error.
Altered function logic

Altered funciton logic to be like tree method ```accumulate_to_ancestor```. Runs significantly faster in tests and returns correct results on the case that @wasade identified as problematic.
Resolves @josenavas's comment
Resolved @wasades' comment on 2508
Update docs.
pep8...
pep8...
doc errors
Merge branch 'master' into tree_method_addition
Merge branch 'master' into tree_method_addition

Conflicts:
	skbio/core/tests/test_tree.py
	skbio/core/tree.py
Merge branch 'master' into tree_method_addition

Conflicts:
	skbio/core/tests/test_tree.py
messed up pep8, fixed with this commit
Resolves asthetic comments

Commit resolves asthetic concerns raised as well as pep8 compliance.
Updated descending_branchlength

@Wasade spotted an error in the function logic. This update should fix this by making sure no node in the dictionary we are creating is referenced by the same name. This means we don't undercount like was possible before.
Merge branch 'master' into tree_method_addition