Add function to escape distinguished names

[tsschaffert]

This adds a function to escape distinguished names, as described in RFC4514.

The use case is to allow building valid DNs from untrusted input, for example if the group name is user supplied like in the following example:

func ModifyGroup(groupName string) {
	groupCN := fmt.Sprintf("cn=%s,ou=groups,dc=example,dc=org", ldap.EscapeDN(groupName))

	ldap.NewModifyRequest(groupCN, nil)
	...
}

[cpuschma]

LGTM! I'll leave the PR open for a few days for others to comment. Thank you for your contribution! 👍

[johnweldon]

LGTM; nice!