Add wrap remote.
IndexEntry: keep a copy of the underlying git_index_entry

The tree entries exist more or less independently of the index they were
retrieved from. The idea behind them is that they can be kept by a
binding without needing to refer to the original anymore, which may
disappear at any moment if the index is modified.

Keep a copy of the data in TreeEntry instead of pointing to the one
retrieved from the index, which is not safe to keep around.
IndexEntry: allow creation of this object

They are not required to belong to a particular index, so it should be
possible to create them at runtime in order to insert them.
Drop Remote_init.
Remove left over declaration of Remote_set_callbacks
IndexEntry: make the attributes writable

When updating entries in an index, it is necessary to modify the
attributes of tree entries. make it possible to do so.
Index: accept adding either a path or an IndexEntry

A path is only useful if we have the file on the worktree. Passing an
IndexEntry allows us to add an entry with arbitrary attributes.
Index: allow writing the tree to a particular repository

Take an optional repository in Index.write_tree() to serialize to a tree
into a particular repository's odb.
Merge remote-tracking branch 'carlos/index-add-entry'
Run tests on travis CI for pypy

Because #209 is done
Preparing for release
Preparing release
Update README
Split the Refspec type to its own file
docs: fix warnings
Update copyright year
utils: allow borrowing a C string

We don't always need our own copy of a C string; sometimes we just need
to parse it. Create py_str_borrow_c_str() which returns a char pointer
to python's internal value string, with which we can avoid an extra copy.
config: borrow the string for lookup and setting

We don't need our own copy of the string, so use the new borrowing
mechanism to use python's underlying string for the key to get/set.
repository: borrow C strings where possible
signature: borrow the name's C string