Compute Limits (#121)

* Compute Limits

This commit adds support for the limits API. It includes the ability
to query limits for the currently scoped user as well as to query the
limits for a specific tenant.

* Clarifying RAM measurement

* Removing ExtractAbsolute. Renaming ExtractLimits to Extract
diff --git a/openstack/compute/v2/extensions/limits/requests.go b/openstack/compute/v2/extensions/limits/requests.go
new file mode 100644
index 0000000..70324b8
--- /dev/null
+++ b/openstack/compute/v2/extensions/limits/requests.go
@@ -0,0 +1,39 @@
+package limits
+
+import (
+	"github.com/gophercloud/gophercloud"
+)
+
+// GetOptsBuilder allows extensions to add additional parameters to the
+// Get request.
+type GetOptsBuilder interface {
+	ToLimitsQuery() (string, error)
+}
+
+// GetOpts enables retrieving limits by a specific tenant.
+type GetOpts struct {
+	// The tenant ID to retrieve limits for
+	TenantID string `q:"tenant_id"`
+}
+
+// ToLimitsQuery formats a GetOpts into a query string.
+func (opts GetOpts) ToLimitsQuery() (string, error) {
+	q, err := gophercloud.BuildQueryString(opts)
+	return q.String(), err
+}
+
+// Get returns the limits about the currently scoped tenant.
+func Get(client *gophercloud.ServiceClient, opts GetOptsBuilder) (r GetResult) {
+	url := getURL(client)
+	if opts != nil {
+		query, err := opts.ToLimitsQuery()
+		if err != nil {
+			r.Err = err
+			return
+		}
+		url += query
+	}
+
+	_, r.Err = client.Get(url, &r.Body, nil)
+	return
+}