Implementing generic parser plugins and documentation

This constitutes a large change in how we will parse different data
formats going forward (for the plugins that support it)

This is working off @henrypfhu's changes.

plugins/parsers/graphite/errors.go

@@ -0,0 +1,14 @@
+package graphite
+
+import "fmt"
+
+// An UnsupposedValueError is returned when a parsed value is not
+// supposed.
+type UnsupposedValueError struct {
+	Field string
+	Value float64
+}
+
+func (err *UnsupposedValueError) Error() string {
+	return fmt.Sprintf(`field "%s" value: "%v" is unsupported`, err.Field, err.Value)
+}