Expense import improvements

Expense import improvements. When the initial version was merged, no resource were available to implement the suggestions made in the MP :

- In hr_expense_accountedge/hr_expense_accountedge.py, It's better to search for the exact tax external reference or have a field linking the taxes that you need. And the module l10n_ca should be listed as a dependence in your __openerp__.py.

- Why not use Python's built-in csv module to produce the tsv in hr_expense_expense._create_csv_report()?

- In hr_expense_expense._get_cur_account_manager(): that logic for getting a comma-separated list of emails is not idomatic and overly complicated. The idiomatic way to do that in Python is to have a list of emails to join into a string and call ','.join(email_list)