Add ldap_escape()

This patch defines a new function and two new constants in ext/ldap:

- `ldap_escape()`
- `LDAP_ESCAPE_DN`
- `LDAP_ESCAPE_FILTER`

Escaping is done according to the relevant RFCs:
http://tools.ietf.org/html/rfc1960 (FIlter)
http://tools.ietf.org/html/rfc2253 (DN)

The function also allows the user to specify a list of characters that will not be escaped.