Can't specify limits on Sequences

When settings limits on a sequence, such as
```yaml
packages:
sequence_demo:
actions:
my_action:
function: main.js
limits:
memorySize: 1024
sequences:
my_sequence:
actions: >
my_action,
my_action
limits:
memorySize: 1024
```
it causes an error:
```
Error: manifest_parser.go [115]: [ERROR_YAML_PARSER_ERROR]: File: [manifest.yml]:
==> yaml: unmarshal errors:
==> line 37: field limits not found in struct parsers.Sequence
```

However, Sequence memory limits can be set via the regular cli:
`wsk action update my_sequence --memory 1024`

Start by reading manifest_parser.go around line 115 and the parsers.Sequence definition, then reproduce the manifest.yml example with a sequence-level memorySize limit. Compare the manifest behavior with the existing `wsk action update my_sequence --memory 1024` command; done means the manifest accepts the limit without the YAML parser error and applies it to the sequence.

Written by the indexing model from the issue text.