Smarter balance assertion dates; handle new ofx fields

Improvements

• Handle the INVEXPENSE field (#79) [Jacob Farkas]
  • Handle the INVEXPENSE field
  • Rename the 'expenses' property to 'invexpense' to match the OFX tag
• handle 'fee' ofx type in investments. [Red S]
• configurable balance assertion dates. [Red S]
• smarter balance assertion dates. [Red S]
  We find the statement's end date from the OFX file. However, banks and
  credit cards typically have pending transactions that are not included
  in downloads. When we download the next statement, new transactions may
  appear prior to the balance assertion date that we generate for this
  statement. To attempt to avoid this, we set the balance assertion date
  to either two days before the statement's end date or the last
  transaction's date, whichever is later.
• needs_update: add --all-accounts. [Red S]

Fixes

• update requirements.txt; also replace == with >= #87. [Red S]
• #85, balance dates were returning function instead of date. [Red S]
• update unit tests to match smart date. [Red S]
• add 'xfer' to ofx types. [Red S]
• make "invexpense" optional with a smart default. [Red S]
• resolved several gotchas with balance assertion dates with ofx files. [Red S]
• #80 document xlrd requirement. [Red S]
  xlrd is optional, and thus not in requirements.txt, but is needed for
  xls files and for testing